The compounds represented by the formula (I) are useful as intermediates for preparing antibacterials represented by the formula (I) but in which B.sup.1 and/or B.sup.2 are a hydrogen or alkali metal atom, e.g., latamoxef or its salt, i.e., B.sup.1 and B.sup.2 are hydrogen or sodium, Ar is hydroxyphenyl and Tet is 1-methyl-1H-tetrazol-5-yl or its derivatives.
When the compound (I) is prepared by chemical synthesis, it is a mixture of (S)- and (R) epimers (I) and (II), and hardly purified by mere crystallization. Then, for preparing the desired antibacterial substance in highly pure state or for storing the intermediate for a long time in a stable form, it is necessary to separate each epimers and then to purify each stereoisomer separately. There has been used a tedious chromatography for this separation, but this method was unsuitable for a large scale production.
The present inventors sought a method for separating these epimers on an industrial scale and found that when a compound represented by the formula (II) is dissolved in ethyl acetate and concentrated in the presence of pyridine to show over 80% of the epimer (II) isomerized to the corresponding epimer (I) and then crystallized as the pure epimer (I). This invention is completed based on this discovery.